Motivation
----------
1) It's required for Walter's work on using -betterC for the
backend conversion:
https://github.com/dlang/dmd/pull/6907
2) We start to run into random failures lately, e.g.
https://github.com/braddr/d-tester/issues/63
https://github.com/dlang/dmd/pull/7569#issuecomment-356992048
3) There's also WIP to move dmd-cxx to 2.076.1:
https://github.com/dlang/dmd/pull/7595
So if we have to bump the minimal dmd version required to build
DMD anyways for (1), let's be clear about it and state it.
Anyone objecting?
